Multi-carrier modulation is known as a modulation/demodulation technique used for radio communication systems realizing multiplex communications. The multi-carrier modulation divides a high-bit-rate data signal (broadband signal) into multiple low-bit-rate data signals that are carried by multiple sub carriers. OFDM (Orthogonal Frequency Division Multiplexing) is a typical multi-carrier modulation in which the frequencies of the sub-carriers are orthogonal to each other, and is adopted in wireless LAN standards such as 802.11a and 802.11g. The sub-carries in transmitted data are closely arranged so as to overlap each other. An FFT (Fast Fourier Transform) circuit of the receiver is capable of separating the sub-carriers from each other.
In OFDM, the receiver has an AFC (Automatic Frequency Control) circuit, which synchronizes the carrier frequencies at the transmitter and receiver sides with each other in order to prevent the sub-carriers from interfering with each other. However, a constant phase rotation is caused in the sub-carriers due to a residual frequency error newly produced during the AFC and phase noise newly applied in frequency conversion. The receiver is thus equipped with a phase tracking circuit for correcting the constant phase rotation. The phase tracking circuit detects the amount of phase rotation using a plot signal that serves as a reference, and corrects the phases of the sub-carriers on the basis of the amount of phase rotation thus detected (see Japanese Patent Application Publication Nos. 2001-53712 and 2001-127730).
More specifically, the conventional phase tracking circuit in the radio receiver averages the amounts of phase rotations detected by using multiple pilot signals included in data in one symbol, and corrects the phases of the sub-carriers using the averaged amount of phase rotation. The above correction may restrain influence of thermal noise. However, there may be great differences in the amount of phase rotation between the multiple sub-carriers in the same symbol in OFDM communications. Particularly, the above problem is conspicuous at frequencies in the millimeter wave range (in the range of 60 GHz). Thus, the averaged amount of phase rotation does not correct the phase rotations of the sub-carriers accurately, and degrades the reception characteristics.